Starting with NSX-T version 3.1 it's a feature to have NSX-T create it's own service account when registering a vCenter Server as a Compute Manager. In the image below you can see the option enabled in the vCenter connection.
When searching for the newly created service account I found that it was not listed in the vsphere.local accounts listing. The next image shows the list of users in the vsphere.local domain but no service account for NSX-T is listed.
To find the service account that was created use PowerCLI to list the vsphere.local domain users. It will show all users including the ones that are hidden from view in the vSphere Client. The command to use is:
Get-VIAccount -Domain vsphere.local | Select-Object Name, Id
The result is the output such as in the next image that shows the service account in the list.